A backseat driver is, in the literal sense, someone who gives directions to the driver from the backseat. Normally, this is considered a bad thing, as most drivers prefer thinking for themselves, ie 'backseat driver' is a derogatory term.
A perfect example of a backseat driver in the literary sense, would be Holly Hunter in Broadcast News, who keeps getting into cabs and telling the taxi drivers which roads to take to avoid traffic etcetera. A good example of a backseat driver in the not-so-literary sense is Clippy, that 'charming' animated paper clip that comes with Microsoft Office as a so-called assistant ("It looks like you're writing a letter...") | 
